This invention relates to disposable electrochemical sensors of the type used for quantitative analysis, for example, of glucose levels in blood, or the like.
Light transmissive electrodes are known in the prior art, however they have not previously been applied to amperometric cells. For example, GB 2 194 112 discloses the use of optically transparent electrodes used to drive a microelectrophoresis cell while laser Doppler velocimetry is used to determine the velocity and micro current motion of charged particles within the sample.
JP, A 05080018 discloses another approach to making transparent electrodes by the use of conductive glass for electrochromic and field emission devices.
JP, A 06310746 also teaches the use and formation of yet another type of conducting transparent electrode formed from the deposition of organic conducting polymers onto a glass slide. This type of electrode is useful in solar energy collection cells. Ullery, in U.S. Pat. No. 4,254,546 also discloses a photovoltaic cell in which the top layer is a light collecting electrode. U.S. Pat. No. 4,782,265 discloses two spaced apart translucent electrodes useful in luminescent cells. However, U.S. Pat. No. 4,782,265 specifically teaches that gold, silver, aluminum, platinum and the like are only suitable for the production of non-transmissive electrodes.
In co-pending applications PCT/AU95/00207, PCT/AU96/00365, PCT/AU96/00723 and PCT/AU96/00724 (the contents of which are incorporated herein by reference) there are described various very thin electrochemical sensors or cells. These cells are by a pair of oppositely facing spaced apart electrodes which are formed as thin metal coatings (for example sputter coatings) deposited on thin inert plastic film (for example 100 micron thick PET). The electrodes are separated one from the other by a spacer of thickness of for example 500 xcexcm or less.
Such cells may be provided with one or more fluid passageways into and out of the sample reservoir whereby the cell may be filled with an analyte and air expelled during filling. In some embodiments the analyte is drawn into the cell by the energy liberated as a reagent contained therein dissolves.
The sensors are, as discussed above, very small and normally contain only small amounts of the liquid sample. Accurate measurement requires that the cell be filled with liquid. Even minute variations in the quantity of liquid in the cell can affect the sensing measurements. It can be difficult for a user to be sure that in use the cell has been uniformly filled with a sample to be analysed.
Further, sensors of the kind under discussion are usually intended to be discarded after use. If a user is distracted after use or prior to disposal it is not always easy for the user to know which sensors have been used and which have not.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
According to one aspect the present invention provides an amperometric electrochemical cell comprising a first insulating substrate carrying a first electrode, a second insulating substrate carrying a second electrode, said electrodes being disposed to face each other and spaced apart by less than 500 xcexcm, and defining a sample reservoir therebetween, wherein at least one of said insulating substrates and the electrode carried thereon includes an electromagnetic radiation transmissive portion in registration with said reservoir.
Preferably, both said insulating substrates and the electrodes thereon include a transmissive portion. Most preferably, the transmissive portion is formed by a conductive metallic coating on the substrate, which is of a thickness such that it is transparent or translucent. Suitable substances for the metallic coating include gold, indium oxide, tin oxide or mixtures thereof. A suitable substrate is PET.
According to a second aspect, the invention provides a method of filling an amperometric cell comprising the steps of:
a) drawing a liquid sample into said cell comprising a first insulating substrate carrying a first electrode, a second insulating substrate carrying a second electrode, said electrodes being disposed to face each other and spaced apart by less than 500 xcexcm, and defining a sample reservoir therebetween, wherein at least one of said insulating substrates and the electrode carried thereon includes an electromagnetic radiation transmissive portion in registration with said reservoir
b) exposing the transmissive portion to electromagnetic radiation
c) monitoring a property of the electromagnetic radiation passing and/or reflected through said transmissive portion
d) comparing said monitored property with a predetermined value indicative of the cell being filled, and
e) continuing to draw the liquid sample into the cell until said monitored property reaches said predetermined value.
According to a third aspect, the invention provides a method of determining whether an amperometric cell is filled with a liquid sample, said cell comprising a first insulating substrate carrying a first electrode, a second insulating substrate carrying a second electrode, said electrodes being disposed to face each other and spaced apart by less than 500 xcexcm, and defining a sample reservoir therebetween, wherein at least one of said insulating substrates and the electrode carried thereon includes an electromagnetic radiation transmissive portion in registration with said reservoir, said method comprising the steps of:
a) exposing said transmissive portion to electromagnetic radiation,
b) monitoring a property of the electromagnetic radiation passing and/or reflected through said transmissive portion, and
c) comparing said monitored property with a predetermined value indicative of the cell being filled.
Suitable forms of electromagnetic radiation include visible, ultraviolet, infra-red and laser light. Daylight is especially preferred. The monitored property can include optical density, wavelength, refractive index and optical rotation.
In preferred embodiments, it is envisaged that the sample will be blood. The electromagnetic property may be monitored inside (for instance with a fibre optical device) or outside the cell, and the electromagnetic radiation may pass substantially directly through the cell or be internally reflected within the cell.
According to a fourth aspect, the invention consists in a method for monitoring an analyte in a liquid sample comprising the steps of:
a) drawing the sample into an amperometric electrochemical cell comprising a first insulating substrate carrying a first electrode, a second insulating substrate carrying a second electrode, said electrodes being disposed to face each other and space apart by less than 500 xcexcm, and defining a sample reservoir therebetween, wherein at least one of said insulating substrates and the electrode carried thereon includes an electromagnetic radiation transmissive portion in registration with said reservoir.
b) exposing the transmissive portion to electromagnetic radiation
c) monitoring a property of the electromagnetic radiation passing and/or reflected through said transmissive portion
d) comparing said monitored property with a predetermined value indicative of the cell being filled, and
e) prior to, simultaneously with or after any one of steps b) to d) applying a potential across the electrochemical cell and measuring the resultant current to detect the analyte.
The method of the above aspect may also further comprise the step of:
f) repeating steps a) to e) until the monitored property reaches the predetermined value.
In one preferred embodiment, the method is repeated on different cells, with blood as the sample and visible light the electromagnetic radiation, until a valid measurement is obtained for blood glucose.
In a fifth aspect, the invention provides an apparatus for determining whether an amperometric cell according to the first aspect is filled with a liquid sample, said apparatus comprising an electromagnetic radiation mans adapted to expose said transmissive portion of said cell to electromagnetic radiation, a monitoring means adapted to monitor a property of the electromagnetic radiation passing and/or reflected through said transmissive portion, and a means for determining whether said monitored property has reached a predetermined value indicative of the cell being filled.
Preferably, said apparatus may also include means to apply potential across the amperometric cell and detect the resultant current. It may also include a validation manes to confirm the cell is filled with a liquid sample.
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Light-transmissive cells intended for spectrophotometric use are well known, However this has not previously been accomplished in a cell wherein the only surfaces suitable for a window are entirely covered by a metal electrode. One skilled in the art will appreciate that whilst the embodiments of the invention are described with respect to light transmissive conductive coatings, such coatings may be also be transparent to some other forms of electromagnetic radiation which are not visible to the human eye.